Difference Between Formula Unit Mass and Molecular Mass

Key Difference – Formula Unit Mass vs Molecular Mass
 

Formula unit mass or formula mass of a compound is the mass of empirical formula of that compound. The empirical formula of a compound is the chemical formula that gives the ratio between different atoms present in that compound in small, whole numbers. In other words, it is the least ratio between chemical elements in a compound. It does not give the actual number of each chemical element. When the mass of this formula is measured, it is known as formula unit mass. The molecular mass of a compound is the mass of a molecule of a particular substance. Most of the time, the terms molar mass and molecular mass are used interchangeably, though they are distinct terms. Molar mass is the mass of a mole of the substance whereas molecular mass is not the mass of a mole of a substance. For simple molecules, formula mass and molecular mass are the same. But for complex compounds, there is a large difference between two values. The key difference between formula unit mass and molecular mass is that the value of formula unit mass is always smaller or similar to the molecular mass of the same substance whereas the value of molecular mass is always larger or similar to the formula unit mass of the same substance.

CONTENTS

1. Overview and Key Difference
2. What is Formula Unit Mass
3. What is Molecular Mass
4. Similarities Between Formula Unit Mass and Molecular Mass
5. Side by Side Comparison – Formula Unit Mass vs Molecular Mass in Tabular Form
6. Summary

What is Formula Unit Mass?

Formula unit mass or formula mass is the mass of the empirical formula of a particular substance. This means, it is the sum of the atomic masses of the chemical elements present in the empirical formula. The empirical formula is the chemical formula of a substance that gives the simplest ratio between chemical elements present in a substance. For example, the empirical formula for C₆H₁₂O₆ (glucose) is CH₂O. Let us consider few examples to understand the concept.

Calculation of Formula Unit Mass

Formula Unit Mass of Simple Compounds

For simple compounds such as NaCl (sodium chloride), the empirical formula and the molecular formula are the same. The value of the formula unit mass is,

(Atomic mass of Na) + (atomic mass of Cl) = (23 + 35.5) amu = 58.5 amu

Formula Unit Mass of Complex Compounds

For complex compounds such as C₁₁H₂₂O₁₁ (sucrose), the empirical formula is CH₂ Then the formula unit mass is,

(Atomic mass of C) + 2(atomic mass of H) + (atomic mass of O) = (12 + {2×1} + 16) amu = 30 amu.

For polymer compounds, formula unit mass is the mass of a repeating unit. A polymer is a macromolecule that is made out of many numbers of small compounds known as monomers. A repeating unit represents the monomer or monomers used to produce the polymer compound. Hence, it is equivalent to the empirical formula of a complex compound.

What is Molecular Mass?

Molecular mass is the mass of a molecule of a particular substance. It is also known as the molecular weight. The molecular weight is calculated as the sum of the atomic mass of all the chemical elements present in the molecule considering the actual ratios between those elements.

Therefore, for large, complex compounds, the value of the formula unit mass is always larger than that of the molecular mass. But for small, simple molecules, both can be the same.

Calculation of Molecular Mass

Let us consider some examples to understand the method of calculation.

Example: Glucose

The chemical formula of glucose is C₆H₁₂O₆. Therefore the molecular weight of glucose is,

6(Atomic mass of C) + 12(atomic mass of H) + 6(atomic mass of O)

= 6(12 amu) + 12(1 amu) + 6(16 amu)

= (72 + 12 + 96) amu

= 180 amu.

Example: Calcium Carbonate

The chemical formula of calcium carbonate is CaCO₃ , which is also the empirical formula of that compound. Therefore, the molecular weight of calcium carbonate is,

(Atomic mass of Ca) + (atomic mass of C) + 3(atomic mass of O)

= (40 + 12 + {3 x 16}) amu

= 100 amu

What are the Similarities Between Formula Unit Mass and Molecular Mass?

• Both Formula Unit Mass and Molecular Mass are weight measurements of substances.
• Both Formula Unit Mass and Molecular Mass have the same unit of measurement.

What is the Difference Between Formula Unit Mass and Molecular Mass?

Summary – Formula Unit Mass vs Molecular Mass

Formula unit mass is the mass of the empirical formula of a particular substance. The molecular mass of a substance is the actual mass of a molecule of a particular substance. The difference between formula unit mass and molecular mass is that the value of formula unit mass is always smaller or similar to the molecular mass of the same substance whereas the value of molecular mass is always larger or similar to the formula unit mass of the same substance.

Reference:

1.Helmenstine, Anne Marie, D. “Formula Mass Versus Molecular Mass.” ThoughtCo, Mar. 25, 2017. Available here 
2.“Molecular mass.” Wikipedia, Wikimedia Foundation, 25 Feb. 2018. Available here  
3.“Empirical formula.” Wikipedia, Wikimedia Foundation, 18 Feb. 2018. Available here  

Image Courtesy:

1.’Alpha-D-glucose Haworth’By Masur (Public Domain) via Commons Wikimedia